[Vizcaster]

Ok back to topic (I'll defer to the moderator's judgment on where this belongs). You can't color sand a metallic finish (for instance autobody metal flake grey - ask me how I know that). One school of thought for other colors is to level the finish to eliminate orangepeel before topcoating with clear, and that has to be done with 400, 600, 600 grit paper (depending on the material you might start higher or leave off earlier - Krylon rattle can users should comment here). This process would actually knock the gloss off, to answer your question, and is not a problem because the topcoats will give back the gloss. It also provides a micro scratch pattern that would give the new finish something to hold onto, which is only necessary if you have a fully cured undercoat or a different type of topcoat. In that case you're adding mechanical adhesion (into the nooks and crannies of the sanded surface) along with chemical adhesion (from the new finish 'burning in' to the compatible old finish). The other method (which seems to be what the TV hotrod guys do) is to simply but a crapload of clearcoat on there and cut it back dramatically by wetsanding after it's cured, then buffing it out to get a shine. I like to do a little of both.

And I have reverted back to dry sanding instead of wet - although you have issues on what to do with the dust and the paper can load up, dry sanding makes it much much easier to see what you're doing so the paper is less likely to burn through an edge (you can also guess how I figured that one out).

I know this is the really boring answer, but you should try a test panel before completing the real thing. Not only will it give you predictable results and help avoid disaster by detecting incompatibilities, it's also good practice so you've got just that bit more bit of experience before tackling the workpiece. Best of luck - there's nothing like hearing somebody say, "You finished that yourself? How did you get it to shine?"